1. Field of the Invention
The present invention relates to a control device for controlling the air-fuel ratio and the ignition timing of an internal combustion engine which can operate with a lean air-fuel mixture.
2. Description of the Related Art
Nowadays, there is considerable interest in designing an internal combustion engine which can operate with a lean air-fuel mixture and thus achieve greater fuel economy. Many improvements have been made in internal combustion engines for that purpose.
For example, U.S. Pat. No. 4,438,741 discloses an improved internal combustion engine having a helical intake port with a straight inlet portion and a helically shaped portion around an intake valve. In that patent, a bypass port is branched from the straight inlet portion and merged into the end of the helically shaped portion of the helical port. A bypass valve is arranged in the bypass port, which is opened when the engine speed and load are high and closed when the speed and load are low. Since a large volume of air can flow into the combustion chamber via both the helical port and the bypass port when the bypass valve is opened, the helical port is free from flow resistance during high load conditions, so that it can be shaped to create a strong swirl in the combustion chamber when the bypass passage is closed. The swirl assists combustion to enable sufficient burning speed for required engine performance even when the air-fuel ratio is relatively lean.
Electronic control devices associated with microcomputers have come into increased use in an internal combustion engines. Such electronic control devices enable control of the air-fuel ratio, fuel injection timing, and ignition timing in response to variable engine operating conditions. This in turn enables the engine to operate with an air-fuel mixture leaner than the theoretical air-fuel ratio when the engine does not require high power.
Japanese Unexamined Patent Publication (Kokai) No. 58-143108 discloses a method of controlling the air-fuel ratio using an oxygen sensor; which outputs a current typically at air-fuel ratios leaner than the theoretical air-fuel ratio and a outputs a voltage at the theoretical air-fuel ratio.
In combustion with a lean air-fuel mixture, it is preferable to simultaneously control the spark timing in relation to the air-fuel ratio. For example, the ignition timing is preferably retarded when the air-fuel ratio is relatively small (relatively rich) since knocking can be prevented by retarding the ignition timing. The ignition timing is preferably advanced when the air-fuel ratio is relatively large (relatively lean) as compared to that when the air-fuel ratio is at the theoretical value so as to prevent the instability of combustion which might occur with the lean air-fuel ratio.